Sorry for the delay, just saw your post, 2oldnslow!
Excuse me if I over-simplify but I'm not sure how technical you are.
Normally, a loudspeaker cabinet is fed a full range signal to it's + and - connections on the back, which delivers the electrical signal to a passive (non-powered) crossover of indeterminate quality (usually a few cheap capacitors which change characteristics over time) which splits the signal into high and low frequency ouputs which are then fed to the respective high and low speaker assemblies.
Different loudspeakers have different crossover frequencies, hence the passive crossover inside each loudspeaker cabinet design is different, there is no real one size fits all design.
It's more sonically efficient to split the full range signal at line level inside the reciever before the main amplification stage.
On professional rigs, this duty is performed in a device called an active crosover, indeed it's not unusual to split the full range line signal into several bands such as sub bass, bass, lo-mid, hi-mid, top, hi-top, etc.
In that way the amplifiers and speakers are matched to produce the best sound possible in each specific frequency range and the energy of each band can be adjusted indefinately.
Back to the domestic reciever, I'm not familiar with your particular Onkyo model, but I'm sure there will be an adjustment entry in the menu somewhere if you select the bi-amp option.
In this way you can select the frequency cutoff point where the reciever splits the line signal between the high and low amplifiers.
As I mentioned earlier, this cutoff point will vary from each model of loudspeaker but such information is easily available from most manufacturer's websites.
Selecting the wrong crossover point will result in a "hole" in the frequency spectrum which is not a good idea.
One last tip : if your Onkyo is fitted with the Audyssey auto setup system, don't be afraid to spend some hours tweaking the settings as the result it arrives at is not always perfect!
I hope this answers your questions!
Just as the point of woofers and tweeters is that one single cone doesn't have the capability to reproduce both low and high frequencies at the same time with any accuracy, the advantage of bi-amping is that there are dedicated amplifier channels for bass frequencies and seperate amplifier channels for high frequencies.
This results in a much cleaner sound as the amps don't work so hard.
Please note that the above explanation is much over simplified, but I think you'll get the idea!